Atom-level interaction design between amines and support for achieving efficient and stable CO2 capture

Amine-functionalized adsorbents offer substantial potential for CO2 capture owing to their selectivity and diverse application scenarios. However, their effectiveness is hindered by low efficiency and unstable cyclic performance. Here we introduce an amine-support system designed to achieve efficient and stable CO2 capture. Through atom-level design, each polyethyleneimine (PEI) molecule is precisely impregnated into the cage-like pore of MIL–101(Cr), forming stable composites via strong coordination with unsaturated Cr acid sites within the crystal lattice. The resulting adsorbent demonstrates a low regeneration energy (39.6 kJ/molCO2), excellent cyclic stability (0.18% decay per cycle under dry CO2 regeneration), high CO2 adsorption capacity (4.0 mmol/g), and rapid adsorption kinetics (15 min for saturation at 30 °C). These properties stem from the unique electron-level interaction between the amine and the support, effectively preventing carbamate products’ dehydration. This work presents a feasible and promising cost-effective and sustainable CO2 capture strategy.

achieve a uniform dispersion of PEI within .A specific amine, PEI-1200, was selected for impregnation due to its optimal molecular size relative to the pore size of the MIL-101(Cr).Additionally, they investigated the CO2 adsorption capacity of PEI-MIL-101(Cr) at various temperature.The cyclic tests were also performed.To elucidate the binding sites of the PEI in the MIL-101(Cr), a washing process with H2O is employed for the PEI-decorated .Following the washing process, it is confirmed that the majority of the PEI is located within the MIL-101(Cr).Furthermore, the authors utilize Density Functional Theory (DFT) calculations to demonstrate that the PEI bonded with the Open Metal Sites (OMS) of the MIL-101(Cr) suppresses the formation of urea.In my opinion, this paper is rejected owing to a lack of novelty to be published in Nature Communications.Therefore, the authors need to clearly explain the novelty of this work in comparison to previous studies (ACS Sustainable Chem.Eng. 2016, 4, 5761;Scientific Reports, 2013, 3, 1859) and additional information is necessary to account for the observed phenomenon.
1.I don't understand how to obtain the PEI-impregnated  shown in Figure 1.In Figure S4, a new impregnation method is introduced.I am curious whether the PEI-impregnated MIL-101(Cr) obtained at different reaction times are indeed acquired through this new impregnation method.If this is correct, it is necessary to include a detailed explanation of this process in the experimental section.2. The authors should compare the amine-impregnated MIL-101(Cr) and present the results in Table 2. 3. What are the adsorption conditions in Fig 2d ?Additionally, both conditions show a gradual reduction in the adsorption curves, suggesting instability in the sample.The reduced capacity raises suspicion of amine leaching or the formation of urea.Therefore, authors need to provide additional explanation on this point.4. I don't believe the urea inhibition mechanism though DFT calculation results only.Therefore, the authors should provide experimental data, demonstrating that the sample does not form urea even when exposed to humid conditions.This additional evidence is essential for a comprehensive understanding of the inhibition mechanism.5. Finally, what distinguishes this paper from other studies?Reviewer #3 (Remarks to the Author): The research topic is important to me because it is not only related to CO2 capture, an important climate change, but also focused on development of a method for increasing CO2 capture capacity and sorbent stability.The research was well done, from both experimental and thermotical perspectives.The stability of sorbent was demonstrated with 100 cyclic tests.The paper is publishable in NC.

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): The authors have well addressed the questions, and therefore I strongly recommend it for publication in Nature Communications.
Reviewer #2 (Remarks to the Author): The revised paper has been much improved and I support its publication as is.

:Furthermore
The amine-functionalized adsorbents have no corrosion or toxic safety concerns.Moreover, they can be easily implemented using existing equipment, ensuring practical industrial applicability.As for the challenges, the cost of MOFs and large molecular amines are generally high than that of the silica-based supports and small amines.Thus, our future focus would be lowering the cost of the adsorbent through synthetic optimization of MIL-101(Cr), its linker, and the large amines, as well as developing more cost-effective alternatives to the support and amines.We have made the following revisions feasible industrial applicability due to operational stability.Further research is needed to expand the range of specialized amine-support systems, mainly focusing on small molecule amines and readily available porous supports."Comment 4: The paper mentions simplification in the DFT simulations to reduce computational complexity, potentially leading to oversimplification of the urea inhibition mechanism.Further clarification on the limitations of the modeling approach and its implications on the validity of the results would be beneficial.How do you ensure the reliability and accuracy of the DFT simulations despite the simplifications made to reduce computational complexity?Response: Thank you for your valuable comment and concern.A PEI-1200 molecule comprises 15 N, 28 C, and 73 H atoms, making its DFT calculations extremely complex and computationally intensive.Therefore, we adopted a simplification approach to reduce computational complexity.Based on the number of nitrogen atoms bonded to hydrogen atoms, PEI can be categorized into three types of amines: categorized into three types and adopts a structure similar to PEI-1200, just with a reduced length compared to PEI.To further demonstrate the urea inhibition, we conducted an in-situ FTIR experiment, comparing the samples with conventional PEI@SiO2 under the same conditions (Supplementary Fig. 11).The FTIR results indicate that the PEI-functionalized MIL-101(Cr) exhibited excellent urea inhibition, providing evidence for strong binding between the amine and Cr sites and further validating the DFT calculation results.Accordingly, we have made the following revisions and explanation.Corresponding revisions: Page 7, Lines 224-227, "To balance computational complexity and accuracy, we simplified the PEI molecule by considering primary, secondary, and tertiary amines (all amine types) as small amine molecules 42 .These molecules then simulated interaction with the Cr site within the super tetrahedron, the fundamental structure of MIL-101(Cr) cages 43 ."Page 9, Lines 264-268, "Additionally, an in situ FTIR experiment was conducted to verify the urea formation inhibition under humid conditions (Supplementary Fig. 11).Unlike the urea formation observed in 55% PEI@SiO2, no urea peaks were detected in PEIfunctionalized MIL-101(Cr), providing conclusive evidence that IACI does not undergo urea formation even under humid conditions 45 ."Supplementary Figure 11 | In situ FTIR spectra of a 55%PEI@SiO2 and b PEIfunctionalized MIL-101(Cr) adsorbents as a function of adsorption temperature at humid (~5% H2O) CO2 stream with the activated sample as the background.the PEI@SiO2 adsorbent exhibited a significant peak increase at 1706 cm -1 , indicating the formation of urea. 5On the other hand, no such substance formation was observed for the PEI-functionalized MIL-101(Cr), suggesting its pronounced resistance to urea under humid conditions.42.Xia, R. et al.Electrochemical reduction of acetonitrile to ethylamine.Nat.Commun.12, 1949 (2021).43.Liang, J. et al.Encapsulation of a Porous Organic Cage into the Pores of a Metal-Organic Framework for Enhanced CO2 Separation.Angewandte Chemie.132, 6124-6129 (2020).45.Li, K. et al.Research on Urea Linkages Formation of Amine Functional Adsorbents During CO2 Capture Process: Two Key Factors Analysis, Temperature and Moisture.The Journal of Physical Chemistry C. the study highlights the innovative nature of the developed amine-support system, it briefly mentions potential future research directions without elaborating on specific areas or challenges that could be addressed.Expanding on these aspects would provide clarity on the next steps in advancing the field of CO2 capture.Can you elaborate on specific areas or challenges that could be addressed in future research to further enhance the efficiency and applicability of amine-functionalized adsorbents for CO2 capture?Response: Thank you for your constructive and valuable advice.In the specific field of CO2 capture, the amine-support system exhibits promise for applications in various areas, such as room temperature biogas (CO2 and CH4), natural gas (H2O, O2, and CO2), and certain flue gases, owing to its efficient adsorption at room temperature and remarkable stability.However, CO2 capture faces a complex environment where factors like temperature, water vapor, and acidic impurities can significantly influence the capture efficiency [Chem.Rev. 121, 12681-12745 (2021); Chem.Soc.Rev. 51, 9340-9370 (2022)], which is also a current focus of our research efforts.We have made the following revisions and discussion.binding sites of the PEI in the MIL-101(Cr), a washing process with H2O is employed for the PEI-decorated MIL-101(Cr).Following the washing process, it is confirmed that the majority of the PEI is located within the MIL-101(Cr).used as a support material for amine loading and demonstrated its CO2 adsorption capacity under various conditions.These studies have significant implications for the application of MIL-101(Cr) in the field of carbon capture.However, there is a paucity of research on amine dispersion and rapid deactivation, which are crucial factors for effective CO2 capture.These issues present a significant challenge for amine-functionalized adsorbents.Our novelty lies in proposing a new design to introduce a collaborative integration between the amine and support for effectively addressing these challenges.In our research, we have successfully demonstrated the penetration of amines through the hexagonal windows in MIL-101(Cr) and their uniform dispersion within the crystal cages, establishing a novel approach known as impregnation of amines into the crystal internalization (IACI).As depicted in Table 1, the amines within the crystals exhibit exceptional CO2 capture properties.Furthermore, we have elucidated a stable mechanism originating from the interaction of amines with Lewis acid sites at the electronic level.This design strategy of the integrated amine-sincerely appreciate the reviewer's thoughtful comments.We acknowledge that our work lacked a clear description of the experimental procedure concerning the adsorbents' different reaction times.In fact, there is a specific filtration step implemented before the methanol evaporation is completed.At a designated time, such as 5 minutes, we terminate the impregnation reaction and promptly filter the MIL-101(Cr) suspended in the amine methanol solution, followed by vacuum drying to obtain the PEI-functionalized MIL-101(Cr) samples synthesized under different impregnation times, as shown in Figure 1.Based on the analysis of BET and TEM results, it is evident that the amines from different reaction times successfully enter the MIL-101(Cr) cages due to their appropriate molecular size and strong interaction with the Cr sites.We have made the following revisions and supplemented details in the experimental section.Corresponding revisions: Page 10, Lines 304-305 (in the experimental section)resembles ours, and they have also reported the cyclic stability of the material under gas desorption conditions.As shown in Table 1, even when the desorption conditions were milder at 110 °C, their materials exhibited significantly higher decay rates, amine consumption, energy consumption, and cycle time.This could be attributed to the fact that although they loaded the amine onto MIL-101(Cr), they did not induce the amine to enter the MIL-101(Cr) cages and interact with the Cr sites and therefore uniform dispersion of amine you for your valuable comment.We realized that we overlooked including the adsorption conditions.In Fig. 2d, the 90 cycles were conducted under pure CO2 adsorption conditions, while the desorption conditions were carried out separately using pure Ar or pure CO2.During the desorption with pure Ar, urea cannot be generated, the reduction in the adsorption curves is attributed to the slight volatilization of PEI-1200 [J.Am.Chem.Soc.139, 3627-3630 (2017)].During the desorption with pure CO2, not only does the amine tend to volatilize, but it also leads to urea deactivation [Chem.Soc.Rev. 48, 3320-3405 (2019)].Although a significant portion of PEI enters the MIL-101(Cr) cages to suppress urea formation by binding with the Cr sites, a small fraction of surface-bound amines still contributes to the slight decay.Overall, as indicated in Table 1, regardless of the desorption atmosphere, the materials exhibit excellent stability (decay rates of 0.11% and 0.18% per cycle in Ar and CO2, respectively).We have made the inhibition, we conducted an in-situ FTIR experiment, comparing the samples with conventional PEI@SiO2 under the same humid conditions (Supplementary Fig. 11).The FTIR results indicate that the PEIfunctionalized MIL-101(Cr) exhibited excellent urea inhibition even in the presence of humidity, providing evidence for strong binding between the amine and Cr sites and further validating the DFT calculation results.Accordingly, we have made factors directly influence material consumption and the feasibility of application.Comparing our materials with the reported PEI(800)impregnated MIL-101(Cr) [ACS Sustain.Chem.Eng. 4, 5761-5768 (2016)] as shown in Table 1, it is evident that our materials exhibit significantly higher stability and lower energy consumption during regeneration.Notably, our study advances existing research in the following way: 1. Integrated cooperation between amine and support.Unlike previous studies that solely focused on either amine or support, our work aims to develop a specialized amine-support system that carefully matched the molecular structures and chemical properties of the amine and support.2. Novel approach of IACI.We have introduced IACI for CO2 capture, where amine molecules directly enter the crystal structure and interact with internal sites, ensuring their uniform dispersion and increased stability within the supports.This approach effectively reduces the expenses associated with amine-functionalized adsorbents by overcoming poor amine dispersion and rapid deactivation during the cycle.3. Stable mechanism at the electronic level.To explain the structure and resistance to deactivation of IACI, we present the initial evidence of a stable mechanism guided by the interaction between amines and Lewis acid sites at the electronic level.4. Outstanding CO2 capture properties overall.Our study introduces a feasible synthesized adsorbent that exhibits exceptional CO2 capture properties, including low reagent consumption, minimal energy input, and short operation cycles.These findings contribute to the wider application of amine-functionalized adsorbents.Addressing the concerns raised by the reviewer, we have made revisions to the paper to better highlight the core concept of our current work. 29, they did not delve into the design of an amine-support system with atomic-level precision to address hindered diffusion and amine deactivation."readers need to recheck the grammatic errors in this paper.e.g., "… uniform dispersions and highly stability" should be "… uniform dispersions and high stability."Response: Thank you so much for your positive comments.Accordingly, we carefully revised the manuscript and sought assistance from a professional academic editing service to help ensure the avoidance of grammatical errors in this paper (the certificate has been uploaded as a supplementary file).

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energy and cost inputs [Nat.Clim.Chang.7, 243-249 (2017)].